1. Field of the Invention
The invention relates to a glenoid component for a shoulder prosthesis. More particularly, the invention relates to a glenoid component having a long off-axis keel and methods and tools for implanting the glenoid component.
2. Description of the Related Art
The human shoulder joint includes two major components, the glenoid component and the humeral component. The glenoid component is the portion of the scapula which forms a socket-like structure for the humeral component. The humeral component is the upper end of the upper arm which forms a ball-like structure which fits into the glenoid component.
Prosthetic shoulder joints are well known in the art and many different glenoid and humeral components have been provided. Various methods and tools have also been provided for implanting a prosthetic glenoid component. A typical state-of-the-art glenoid component is disclosed in U.S. Pat. No. 5,489,310 to Mikhail which also discloses methods and tools for implanting the component.
The typical glenoid component includes a circular body having a concave lateral articulating surface and an opposing medial surface with a stem extending along a central axis from the medial surface. The glenoid component is installed after the scapula is resected to remove the natural glenoid and a cavity is formed in the neck of the scapula to receive the prosthetic glenoid component.
The methods and tools described in the ""310 patent are designed to permit installation of the glenoid component with a minimal amount of bone removal required in the preparation of the scapula. Utilizing the methods and tools of the ""310 patent, different sizer guides are used to measure the size of the natural glenoid cavity in order to choose a prosthetic component of proper size through trial and error. The sizer guides each have a plurality of pointed members and a central hole. After the proper size has been determined, the corresponding sizer guide is hammered into the bone of the scapula and the central hole in the sizer guide is used as a drill guide to drill a 15-30 mm hole in the scapula.
A reamer of appropriate size is then chosen based on the size of the sizer guide previously chosen. The reamer has a symmetrical head with a plurality of tapered cutting blades and a peripheral stop surface. The previously drilled hole is used as a center guide for the reamer. The reamer is used to create a cavity in the scapula in which the prosthetic glenoid component will be installed. After the cavity has been created, the circular glenoid component is installed in the cavity, with or without the use of bone cement.
Despite the numerous advances in the designs of glenoid components and the methods and tools used for their installation, these prostheses still lack the stability and strength of natural healthy glenoid components and the relative positions of the prosthetic glenoid and humeral components most often does not provide the proper soft tissue balance.
It is therefore an object of the invention to provide a prosthetic glenoid component which more accurately copies the function of a natural glenoid component.
It is also an object of the invention to provide prosthetic glenoid component which is more stable than existing prosthetic glenoid components.
It is another object of the invention to provide a prosthetic glenoid component which, when implanted, provides proper soft tissue balance between the glenoid component and the humeral component.
It is still another object of the invention to provide a glenoid component and methods and tools for its installation which achieve the aforementioned objects and which minimize the risk of damage to the posterior scapula.
In accord with these objects which will be discussed in detail below, the glenoid component of the present invention includes an oval body having a concave lateral articulating surface and an opposing medial surface with a keel extending from the medial surface.
According to one aspect of the invention, the keel extends along a line which is anteriorly offset from the center line of the concave surface and is not collinear with any radius of the concave surface.
According to another aspect of the invention, the keel is substantially triangular or trapezoidal in shape and functions like an I-beam to provide greater support for the glenoid component when attached to the scapula.
According to a presently preferred embodiment, the medial surface of the glenoid component has ridges and the keel has a central hole, both of which enhance fixation of the component to the scapula.
The design of the glenoid component of the invention is based in part on the observation that the centerline of a natural glenoid component is not collinear with the centerline of scapula bone mass. It was thus hypothesized that the symmetrical glenoid components with central stems and the methods and tools for their installation result in the affixation of the component to a portion of the scapula which has reduced bone mass. It is believed that this is the cause of reduced stability, damage to the posterior scapula, and imbalance in the soft tissues between the glenoid and humeral components.
According to further aspects of the invention, new methods and tools are provided for the implantation of the novel glenoid component. In particular, sizer drill guides are provided with an additional alignment arm which assures that the anterior/posterior angle of the pilot hole is approximately 65 degrees. This angle was discovered to be the optimal angle in a series of unpublished cadaver studies.
According to other aspects of the invention, an improved reamer tool is provided which allows the practitioner to attach a reamer head of selected size to the reamer rather than choosing among several reamers of different size at the time of the procedure.
In order to provide the proper hole in the scapula for the keel of the glenoid, a new keel drill guide is provided which locates three drill guide holes in a plane which is offset approximately 2 mm anterior of the pilot hole. One of the drill guide holes is a central hole and the other two holes parallel therewith. A new broach tool is provided for connecting the three holes and forming a triangular cavity for the keel.
The methods of installation according to the invention include selecting the appropriate sizer drill guide in a conventional way, using the sizer drill guide of the invention to drill a pilot hole in the center of the glenoid, but with an A/P angle of approximately 65 degrees, using the new glenoid reamer to resect the glenoid cavity around the pilot hole, using the new keel drill guide to drill three holes which have axes aligned in a plane which is anteriorly offset from the pilot hole, and using broach tool to connect the three holes and form a triangular cavity for the keel. The glenoid component is then installed with bone cement and is placed to tilt slightly downward (up to approximately 5 degrees) in order to reduce superior migration of the humeral head.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.